Some mobile radio standards require additional amplitude modulation, as well as pure phase modulation and/or frequency modulation, in an upgraded version. The extension to the GSM mobile radio standard, by way of example, thus uses a modulation type which codes information both in the phase and in the amplitude. Examples of a modulation method such as this are the 8-PSK (8 phase shift keying) modulation type, as is used in the GSM-EDGE mobile radio standard. Another example is QPSK modulation methods (quadrature phase shift keying modulation). These are used in the third-generation mobile radio standards. A combination of pure phase modulation and/or frequency modulation with amplitude modulation allows an increase in the transmission speed.
Fundamentally, however, transmitter assemblies for mobile radios have to cope with a number of modulation types, for example pure phase modulation such as the GMSK modulation that is used for the GSM mobile radio standard, as well as combined amplitude and phase modulation, such as 8-PSK for GSM-EDGE.
A polar modulator allows data which is amplitude-modulated or phase-modulated at the same time to be transmitted in a manner which is insensitive to external disturbance influences and can be implemented very efficiently.
Every radio-frequency signal (R(t)) with any form of modulation can be represented in polar coordinate form as follows:R(t)=A(t)*cos(ωt+φ(t)
In this case, A(t) represents the amplitude information, which varies over time, φ(t) represents the phase information, which varies over time. The amplitude A(t) and the phase φ(t) thus contain the payload information. A polar modulator is designed for modulation of the amplitude and of this phase.
FIG. 4 shows a block diagram of a transmitter for a signal based on a mobile radio standard, with the signal containing the payload information. The transmitter contains a phase locked loop which modulates the required phase information directly onto the radio-frequency signal. The output signal S(t) from the phase locked loop 1 can thus be described by the function S(t)=f(ωt+φ(t)). In the case of pure phase modulation, this signal is passed directly via a preamplifier 13 and a switch 14 to the output amplifier 15. The output amplifier 15 amplifies the modulated signal to the required level.
In the case of amplitude and phase modulation of the output signal, in contrast, switching takes place to a second transmission path. This is done by switching the switch 14, which connects the input of the power output stage 15, to the output of an amplifier 12. A mixer 11 and a controllable amplifier 12 are activated at the same time. The amplifier 12 is connected on the input side to the output of the mixer 11, which operates as an amplitude modulator and which is supplied with a signal S(t), which is phase-modulated on the input side. After amplitude modulation A(t) of the phase-modulated signal emitted from the phase locked loop 1, this signal is preamplified in a suitable manner by the preamplifier 12, and is supplied to the power output stage.
Thus, in this conventional solution, two different signal paths are provided for the pure phase modulation and combined amplitude and phase modulation. This makes it possible to comply with the possibly different noise and distortion requirements for the various mobile radio standards.
For example, the upper signal path with the amplifier 13 is thus designed for particularly low-noise amplification, but does not provide good linearity for this purpose, since this is of secondary importance for pure phase modulation. In contrast, the lower signal path for combined amplitude and phase modulation may be designed to be less noise-free, but requires a very linear transmission response. Furthermore, for the 8-PSK modulation type, it must be possible to adjust the required output power at the input of the output stage power amplifier 15 over a wide range of about 45 dB. This can be achieved by a variable amplifier 12, whose gain is adjustable and which is connected downstream from the mixer 11, which operates as an amplitude modulator.
However, amplifiers whose gain factor can be varied continuously, so-called VGAs, have the disadvantage that the distortion rises considerably when their gain is reduced, that is to say at low output power levels. This distortion can be reduced only by means of a large bias current through the amplifier, although this makes the efficiency worse and increases the overall power consumption. Furthermore, the space occupied by the two signal paths to be provided is increased, and parasitic effects must be taken into account in the respective signal path which is switched off and is not being used.